New LUX experiment: No dark matter in this corner

Three months of data rule out possible detections in other experiments.

Dark matter is perhaps the most frustrating substance we know. Invisible to all forms of light, we detect its presence through its gravitational influence alone. That's insufficient, though: physicists and astronomers alike would like to know what it is. Yet so far, we're better able to say what it isn't: it's not the stuff of normal matter (electrons, quarks) or other particles we know about, like neutrinos.

The process of figuring out what dark matter isn't received a major push today with the announcement of the first three months of data from the Large Underground Xenon (LUX) detector. Unfortunately, LUX failed to find any dark matter, ruling out some possible detections by other experiments.

Experiments attempting to detect dark matter focus mainly on one possible class, motivated by particle physics: WIMPs, or weakly interacting massive particles. The "weak" part of the name originally referred to the weak force, one of the four known fundamental forces in nature. However, WIMPs may or may not actually interact via the weak force—we know for certain dark matter interacts gravitationally, but whether it interacts with ordinary matter via any other force is currently unknown.

However, the gamble is that dark matter does interact in a limited way with normal matter, and detectors have been designed with that in mind. LUX consists of 368 kilograms of liquid xenon cooled to -110°C and surrounded by a tank of water. The whole apparatus is housed nearly 1500 meters (4,850 ft) underground in the old Homestake gold mine near Lead, South Dakota. The depth of the mine provides a lot of shielding against cosmic rays—high energy particles from supernovae and other astronomical sources—that could otherwise swamp the detector. WIMPs, on the other hand, should pass through the rock and reach the detector without much in the way of interaction.

Liquid xenon is much denser than water, so it provides an additional measure of shielding. Any particle piercing through to the center of the tank has likely avoided interacting with anything until that point. When such a particle collides with the nucleus of a xenon atom, it produces a recoil, emitting photons and electrons. Detectors lining the outside of the tank amplify those signals. Comparisons between the photon and electron timing provide a three-dimensional reconstruction of where the collision occurred and the energy involved. Together, these data reveal the mass and electric charge of the particle, telling researchers whether it could be dark matter or (more likely) something ordinary.

Particle masses

Even a very massive elementary particle has a small mass in terms of the measures we use in daily life: grams or kilograms (or slugs, if you want to get all American about it). Therefore, particle physicists frequently use the famous equation E = mc2 and write masses in units of energy instead. One electron volt (eV) is 1.783×10−36 kg; a proton's mass is about 0.94 GeV and the Higgs boson's mass is about 125 GeV.

LUX is designed to be sensitive to WIMPs across a wide range of possible mass values, overlapping the ranges other detectors are searching. Many theoretical models, including those motivated by supersymmetry (SUSY), predict "high-mass" WIMPs: those with masses greater than 35 billion electron volts (35 giga electron volts, or 35 GeV). However, the Cryogenic Dark Matter Search (CDMS) experiment in Minnesota found small but tantalizing hints of particles with masses around 8.6 GeV, which would fall into the "low-mass" WIMP regime.

This is why the new LUX results are interesting, despite showing nothing. For high-mass WIMPs, LUX has roughly twice the sensitivity of other detectors, but it also can detect particles in the low-mass regime. If the 8.6 GeV WIMPs seen in CDMS exist, three months of LUX operation should have found thousands of them. Yet LUX saw nothing. While it's still possible the CDMS particles could be real, they must interact in a way specific to the solid silicon-based detectors at CDMS and avoid collisions with xenon atoms—an unlikely possibility.

Given the increased sensitivity at LUX, the most popular prediction for a WIMP—the so-called lightest supersymmetric partner (LSSP )—is not looking good either. Depending on the particular model, the LSSP should have a mass greater than about 10 GeV, and LUX should have detected many of them. While the absence of particles is consistent with prior dark matter experiments, LUX's sensitivity makes it easier to say that high-mass WIMPs probably don't exist.

Of course, this result isn't a definitive statement on the existence of dark matter, but it does put stronger constraints on what its identity could be. Dark matter could still lurk at lower mass ranges, or it could be non-WIMP in nature. The latter possibility includes the disturbing prospect that dark matter doesn't interact directly with normal matter at all, except through gravity, dooming every current detection scheme to failure.

LUX will continue operating until 2015, and its planned successor—LUX-ZEPLIN, or LZ—should increase sensitivity to the point where even very rare dark matter collisions can be spotted.

149 Reader Comments

A singularity violates the Pauli exclusion principle, among other things; two things being in the same place at the same time. Well, it is my belief that no part of general relativity is ever violated; it all works together, or none of it works at all.

And if you attempt to cheat, as a black hole does, there will be consequences, because above all, mass and energy will be conserved. (okay not really above all; all the laws of the universe are equally relevant)

I don't think we can not understand a singularity, but then set that problem aside, and go on to continue to expand our knowledge of the universe in general, and relativity in particular. I think science is somewhat stuck, until we can explain that one. And when we do, I think it will explain a lot of things, and open a lot of doors.

"... While current observations do not yet provide a direct model-independent measurement of the speed of gravity, a test within the framework of general relativity can be made by observing the binary pulsar PSR 1913+16. The orbit of this binary system is gradually decaying, and this behavior is attributed to the loss of energy due to escaping gravitational radiation..."

That's the whole meat of your link right there. The observed decay in the received signal is attributed to light speed 'limited' gravity.

Interesting, but hardly definitive. In case you didn't get it, I was disagreeing with the premise of that article, since the question is in no way solved in science as of yet.

I'm afraid it's still fair game for discussion.

I think you sort-of covered this in your reply to wyrmhole but what you are saying is not that in the theory of General Relativity gravitational effects travel faster than c, but that General Relativity is wrong. Because under General Relativity gravitational effects travel at c. There is no debate about this and c as a limit is absolutely fundamental to GR. You can't pick and choose the parts of GR that you like and discard the others - the theory falls apart. If you would like to re-formulate GR without c, or come up with an entirely new theory of gravity, then by all means go for it, but whatever it is has to at a minimum do everything that GR does.

Here's a question for you. If indeed the edges of galaxies are being acted upon by a much larger cloud of dark matter surrounding them, then this dark matter cloud must extend far past the boundaries, I guess, by order(s) of magnitude. So a galaxy's dark matter cloud dwarfs the galaxy itself in size, in order to give the almost even rotational rates from near-center to edge. Given that observed rotational rate, and the necessary mean distance of these wimps, does that not imply that wimps would be travelling at far faster than the speed of light? In order to gravitationally pull the edges at the observed rate, it would seem to be so. Otherwise, wimps would have to truly be very heavy, and distributed in some sort of ring fashion around the disk, which we have not observed. And even then, they would be travelling at a significant fraction of the speed of light, would they not?

Yes, in fact if you read about the structure of galaxies the dark matter halo is far out there beyond virtually all of the conventional mass of the galaxy. It forms basically a vast hollow sphere around the center of the galaxy. In essence what happens is most of the DM's temperature (IE velocity) is fairly homogenized, probably by gravitational-only self interaction, leading to most of it occupying a specific range of orbits. The outer parts of the galactic disk move at a high radial velocity (higher than what would be needed to orbit the conventional mass of the galaxy), while the central parts, being further from this halo, experience less added gravity and suffer less of a velocity anomaly.

Quite. Except, I would say, when it comes to predicting the magnetic field in relation to the electric field. It is an everyday, low velocity relativistic effect and that may be why it makes more intuitive sense.

The nonexistence of dark matter would be shocking. It would mean that no stable fundamental particle more massive than a down quark exists in Nature, that all stable particles interact with the Z boson, and all but the neutrinos interact with the photon and Higgs. That's truly incredible. As soon as you allow that, ok, there may be a single stable particle of significant mas unknown to mankind, you would have a Universe filled with warm/cold dark matter.

I don't think it helps out on the naturalness of the Standard Model particles (but who knows), but at least it would dilute the amount of weirdness of the universe.

Maybe I should withdraw my former characterization of SM particles and claim that so far DM seems much less weird than "ordinary", known, particles...

I think the observation of the rotation of galaxies is an explanation, to bring things full circle. The rotation of the edge of a galaxy should be about 50,000 years behind the center, yet it's not; it rotates at almost the same velocity as the center, and pretty much the same as near the center. This should be impossible.

My explanation for this is that the singularity at the center of the black hole deforms space-time as a result of 'violating' GR. Deforming space-time means deforming gravity. What we are witnessing in the rotation, is the deformed gravity / deformed space being carried around this super-mass in something akin to frame dragging. Perhaps I was wrong in never being able to observe it; we plainly see the 'incorrect' rotations of almost all galaxies.

It is fair enough to say that the presumed singularity in the galactic SMBH violates GR, because a singularity would violate most anything. The recent spat about firewalls shows that the prediction that an observer falling in over the much further out (in SMBHs) event horizon would still see flat space is still arguable. (But the consensus seems to have come down on that flat space continues a fair bit inside a SMBH.)

However as soon as you are a Planck width or so outside the event horizon, you should see no GR violations. Hence "deformed gravity being carried around" outside of the SMBH doesn't work. [ http://en.wikipedia.org/wiki/Membrane_paradigm ; key is that "results of the membrane paradigm are generally considered to be "safe"."]

I think the observation of the rotation of galaxies is an explanation, to bring things full circle. The rotation of the edge of a galaxy should be about 50,000 years behind the center, yet it's not; it rotates at almost the same velocity as the center, and pretty much the same as near the center. This should be impossible.

My explanation for this is that the singularity at the center of the black hole deforms space-time as a result of 'violating' GR. Deforming space-time means deforming gravity. What we are witnessing in the rotation, is the deformed gravity / deformed space being carried around this super-mass in something akin to frame dragging. Perhaps I was wrong in never being able to observe it; we plainly see the 'incorrect' rotations of almost all galaxies.

It is fair enough to say that the presumed singularity in the galactic SMBH violates GR, because a singularity would violate most anything. The recent spat about firewalls shows that the prediction that an observer falling in over the much further out (in SMBHs) event horizon would still see flat space is still arguable. (But the consensus seems to have come down on that flat space continues a fair bit inside a SMBH.)

However as soon as you are a Planck width or so outside the event horizon, you should see no GR violations. Hence "deformed gravity being carried around" outside of the SMBH doesn't work. [ http://en.wikipedia.org/wiki/Membrane_paradigm ; key is that "results of the membrane paradigm are generally considered to be "safe"."]

I would hope that the singularity only appears to a present-day observer to violate GR, and that there turns out to be an explanation. One that preserves mass-energy, exclusion, c, everything. In that case, I think it will be found that no actual singularity exists in the observable universe, and what appears to be one, is covered by an as yet undiscovered extension to GR. Something that explains where the mass-energy; where the infinity part, is 'going'.

As far an observer falling into an event horizon, I, like everyone else, have thought about this many times, but I really don't have an intuitive sense for it. And for the membrane model, I certainly don't have any dispute with it. But isn't the point of it to avoid attempting to model the singularity directly? It seems to me, that if the gravity itself was an issue, that you would have died of heart failure and crushed bones long before approaching any horizon, which you would then enter as hadrons I guess.

But I don't think it's supposed to work that way; the space itself is deformed, but you wouldn't experience any extra gravity, from your reference frame, while falling in. (we are assuming you had an invincible x-ray shield during your approach btw) So I wouldn't expect one to see any GR violations, right up until they smacked into the singularity at the center. But even that is an academic exercise, isn't it, because it would take basically forever to actually reach the center, wouldn't it?

I did say "deformed gravity being carried around", right after I corrected the other guy for it, I then went and butchered it myself. I will only plead that it is a difficult concept, both, to grasp, and to describe in words. Concisely: the singularity is forcing mass-energy into another dimension. Thus, there is no actual singularity, and exclusion and GR is preserved. While this mass-energy is being held in this other dimension by the incredible gravity of the black hole, it can't be hit with EM radiation, and thus can't be observed directly.

But this disappeared mass still has mass, and that mass, apparently, still generates gravity, even from this sequestered location. I use the term location here, obviously, somewhat loosely. And this mass, and it's gravity, rotates around the black hole with the rest of the galaxy. I still think the concept of frame dragging is somewhat applicable here.

Here's a question for you. If indeed the edges of galaxies are being acted upon by a much larger cloud of dark matter surrounding them, then this dark matter cloud must extend far past the boundaries, I guess, by order(s) of magnitude. So a galaxy's dark matter cloud dwarfs the galaxy itself in size, in order to give the almost even rotational rates from near-center to edge. Given that observed rotational rate, and the necessary mean distance of these wimps, does that not imply that wimps would be travelling at far faster than the speed of light? In order to gravitationally pull the edges at the observed rate, it would seem to be so. Otherwise, wimps would have to truly be very heavy, and distributed in some sort of ring fashion around the disk, which we have not observed. And even then, they would be travelling at a significant fraction of the speed of light, would they not?

Yes, in fact if you read about the structure of galaxies the dark matter halo is far out there beyond virtually all of the conventional mass of the galaxy. It forms basically a vast hollow sphere around the center of the galaxy. In essence what happens is most of the DM's temperature (IE velocity) is fairly homogenized, probably by gravitational-only self interaction, leading to most of it occupying a specific range of orbits. The outer parts of the galactic disk move at a high radial velocity (higher than what would be needed to orbit the conventional mass of the galaxy), while the central parts, being further from this halo, experience less added gravity and suffer less of a velocity anomaly.

Still seems to me, that this vast hollow halo, would have to be rotating at quite a significant velocity, given it's distance; and as a group, not homogenized; to account for the observed radial velocity.

Still seems to me, that this vast hollow halo, would have to be rotating at quite a significant velocity, given it's distance; and as a group, not homogenized; to account for the observed radial velocity.

I don't have any idea what the motion of the DM particles themselves has to do with it. If you have a fairly regular shell of DM around the galaxy it will exert a fairly regular gravitational force on the visible matter. If the orbits of the DM particles are within that shell then their individual motion just doesn't matter. Also, remember, distant orbits are pretty low velocity. It all seems quite mundane really, once you accept that the DM is there no significant dynamical problems come up. Your galaxy is stable and everything just works.

But that's not what we observe in a galaxy; it would be as if Neptune keeps in line with Earth, which would mean it's travelling far faster than Earth. And this proposed halo, that's really far out, in our solar scale model it would be light years from the sun, would have to be rotating along with Earth and Neptune, if it is what's responsible for pulling Neptune faster than it would otherwise go.

What I haven't seen, and I have looked, is the proposed mechanism that this cloud of relatively stationary dark matter uses to accelerate the edge of a galaxy to the velocities we see. Mass and gravitational pull from outside the galaxy would not by itself have an accelerating effect. In fact, unless the DM mass/gravity were in motion, and pulling the edge of the galaxy along, it would have the opposite effect; slowing the edge of the galaxy down even further than it's distance from center would suggest.

But that's not what we observe in a galaxy; it would be as if Neptune keeps in line with Earth, which would mean it's travelling far faster than Earth. And this proposed halo, that's really far out, in our solar scale model it would be light years from the sun, would have to be rotating along with Earth and Neptune, if it is what's responsible for pulling Neptune faster than it would otherwise go.

What I haven't seen, and I have looked, is the proposed mechanism that this cloud of relatively stationary dark matter uses to accelerate the edge of a galaxy to the velocities we see. Mass and gravitational pull from outside the galaxy would not by itself have an accelerating effect. In fact, unless the DM mass/gravity were in motion, and pulling the edge of the galaxy along, it would have the opposite effect; slowing the edge of the galaxy down even further than it's distance from center would suggest.

What I'm saying is that in a typical orbital system, like the solar system, the velocity of distant objects like Neptune is indeed low. This is because they feel only a weak gravitational attraction from the distant Sun (and all the rest of the mass of the SS is trivially small and can be safely ignored). There is a specific curve that you can draw for this, which is determined only by the value of G (and scaled by the magnitude of the central mass, IE the Sun). When the mass involved is NOT a point mass, then you can have differently shaped curves. The curves that galaxies exhibit is consistent with a massive halo of matter surrounding the galaxy.

MOTION would have NO EFFECT. It makes no difference at all and is irrelevant. Yes, point masses will of course perturb the motion of other matter when they pass nearby, but DM isn't point masses, and the effect will not be to 'drag along' the other matter. Trust me, this stuff has been extensively modeled and studied by experts in dynamics, it works. http://en.wikipedia.org/wiki/Galaxy_rotation_curve for instance provides a basic explanation of the subject.

Sorry, but you guys really got my juices going, so I got a little more. Hopefully the thread is old enough by now.

I tried to cover the 'space doesn't travel' thing as best I could, which I did pretty terribly.

If there does turn out to be a quanta of gravity; which they have already named gravitons; then obviously, I am completely wrong in all aspects. Such a particle would be subject to the fundamental laws all particles and waves that propagate through the universe are bound to. Gravitational effects would propagate and all that.

But propagate through what? Just travel through free space until it smacks into something? And to a graviton, free open space would be all space, as it obviously passes through mass far easier than a neutrino. In the case of the neutrino, a neutrino simply misses; it's just that tiny. The graviton, on the other hand, hits each and every particle in the universe dead center; it never misses. Every hadron, photon, whatever; is bound, during it's entire existence to, well, every other particle and wave in the observable universe.

For one thing, that's just a whole lot of field lines crossing. The concept seems implausible to me. (unless there was no big bang that that's the source of the CMB. did i say that out loud?)

Here's another issue. GR doesn't specifically predict gravity travels at light speed as a fundamental law. It's when you calculate the propagation of gravitational effects using GR, that you get, surprise, light speed. All that does is validate (again) general relativity. Anything that can be measured, will never be measured above the speed of light. GR is both a blessing and a curse that way. And yes, that implies that we're measuring the wrong thing; that we're basically measuring the gravitational constant.

Which brings up another issue, the 17 or so arbitrary constants that are supposedly fundamental to the universe. Am I alone in thinking that none of those are going to stand the test of time? If it turns out that there really are just fundamental things like 'that's the way it is'; then maybe we should just thank the Creator and quit bothering with science, since we have discovered almost everything. Heh. There's a reason for everything, we just don't know most of them at the moment. So in the context of a subject like this, it's not just okay; it's somewhat necessary to call the fundamentals into question.

So then, how does the pull of the Sun 'get to' the Earth, if it's not travelling? Well, if it's not traveling, then it's not pulling. It feels like it's pulling. But that's just the shape of the space we're standing in at the moment, it's curved, and the direction of that curve is 'downhill', towards the Sun. That's the general curve; that, of course, is on the much larger slope of the Galaxy, and those otherwise even slopes are perturbed by, well, everything else that exists, each of which distort space in an amount corresponding to it's mass.

I did it again; I said 'distort space'. I think that's the wrong way to look at it. It implies that the space was already there, and I said as much above, which I then contradicted with 'gravity is space'. If the space was already there, then indeed, the gravity would have to travel across it. But the fact is, as hard as it is to comprehend, that space, and time, and gravity; are the same single thing. We can model different parts of in GR, and validate them against themselves, which gives us an intuitive, but incorrect, sense that we can consider them independently.

And so here's the big implication. If gravity comes from objects in space, then space also comes from objects in space, as does time. Therefore, space-time does not exist anywhere there is not mass causing it to exist. I use the word 'cause' there loosely; I may very well have that backwards. But I have to use words as best I can, so for the moment, I'm going with; mass-energy causes space-time.

That solves your travelling gravity problem: the space takes the shape it does as it's created; and it's created in real-time, right in the middle of all the other spaces that were already there. The words we use to describe the shape of that space, are gravity and time.

...MOTION would have NO EFFECT. It makes no difference at all and is irrelevant. Yes, point masses will of course perturb the motion of other matter when they pass nearby, but DM isn't point masses, and the effect will not be to 'drag along' the other matter. Trust me, this stuff has been extensively modeled and studied by experts in dynamics, it works. http://en.wikipedia.org/wiki/Galaxy_rotation_curve for instance provides a basic explanation of the subject.

Yeah, I'm familiar.

"If disc galaxies had mass distributions similar to the observed distribution of stars and gas, the rotation curves velocities should decline at large distances (dotted line A in illustration) in the same way as do other systems with most of their mass in the centre, such as the Solar System or the moons of Jupiter, following the prediction of Kepler's Laws."

I don't think I need to link Kepler for you. The DM has to be in rotation, right along with the rest of the galaxy, if it is a part of that system.

In this way, I guess I'm not as caught up on DM theory as I thought. I've sort of assumed, that the DM was always assumed to be in motion.

But you're right, in the sense that I didn't think that a rotating halo could be responsible for pulling the edge along, in the way that we observe. No matter how big and massive the halo, we should still see the visible edge slowing down somewhat. But instead, we see the very outermost star of a galaxy being the fastest, except for the very central stuff, which is obviously being accelerated prior to falling in.

The only way I can see it working is if the DM cloud rotates faster than the galaxy, and (where you have a problem also); each particle exerts it's own independent pull on the visible edges, and evenly all the way through to the center. It should be an even pull, no matter the motion of the DM, unless it was relativistic in some way. And also of course, the problem of how in the galaxy would the DM rotate faster than the general system.

But if the DM is a gravitationally bound component of the system, then it has to rotate with that system. But I'm going to read up a little more, thanks.

...MOTION would have NO EFFECT. It makes no difference at all and is irrelevant. Yes, point masses will of course perturb the motion of other matter when they pass nearby, but DM isn't point masses, and the effect will not be to 'drag along' the other matter. Trust me, this stuff has been extensively modeled and studied by experts in dynamics, it works. http://en.wikipedia.org/wiki/Galaxy_rotation_curve for instance provides a basic explanation of the subject.

Yeah, I'm familiar.

"If disc galaxies had mass distributions similar to the observed distribution of stars and gas, the rotation curves velocities should decline at large distances (dotted line A in illustration) in the same way as do other systems with most of their mass in the centre, such as the Solar System or the moons of Jupiter, following the prediction of Kepler's Laws."

I don't think I need to link Kepler for you. The DM has to be in rotation, right along with the rest of the galaxy, if it is a part of that system.

In this way, I guess I'm not as caught up on DM theory as I thought. I've sort of assumed, that the DM was always assumed to be in motion.

But you're right, in the sense that I didn't think that a rotating halo could be responsible for pulling the edge along, in the way that we observe. No matter how big and massive the halo, we should still see the visible edge slowing down somewhat. But instead, we see the very outermost star of a galaxy being the fastest, except for the very central stuff, which is obviously being accelerated prior to falling in.

The only way I can see it working is if the DM cloud rotates faster than the galaxy, and (where you have a problem also); each particle exerts it's own independent pull on the visible edges, and evenly all the way through to the center. It should be an even pull, no matter the motion of the DM, unless it was relativistic in some way. And also of course, the problem of how in the galaxy would the DM rotate faster than the general system.

But if the DM is a gravitationally bound component of the system, then it has to rotate with that system. But I'm going to read up a little more, thanks.

I never said, or meant to imply, that DM doesn't move. It moves in the same ordinary gravitationally determined way that any other matter moves. I'm just saying that if you have a halo, which is a 'cloud' of DM particles, all in distant orbits around the galactic center, with no preferred orientation (IE they aren't only in the plane of the galactic disc, then the net effect of their gravitation on the rest of the galaxy's components really isn't changed by their own motion. Its a homogeneous assortment of distant orbits at all angles, so the resulting gravitational field is symmetric.

I have no idea why you insist on bringing rotation into this, it simply doesn't matter, there's no need for the DM to 'rotate faster than the rest of the galaxy', and as for 'each particle exerts an independent pull', of course it does, this is the way gravity works. The total overall gravitational field is simply the sum of the gravitational fields of all the component bits of matter. This is no different from the way the gravity of the Earth and Sun work, they are the sum of the gravitation of every fundamental particle in each body. In GR terms its a bit more complicated than this and there are some other factors involved in the stress-energy tensor, but galaxies are actually pretty non-relativistic, which is why MOND has only recently bothered to be extended into TeVeS because basic Newtonian MOND is good enough to model galaxy rotation curves and explore the possibilities. Anyway, the point is with a massive symmetrical halo you can treat it as a rigid non-rotating body to a first approximation. Its composition as individual particles and its rotation are not really terribly relevant to the galaxy rotation curve problem.

The issue of course is that for OTHER situations, things like the Bullet Cluster, the characteristics of the DM and its motion are quite important and things like MOND entirely break down, they simply have no explanatory power. Any other theory that is effectively some "gravity works differently" theory is also going to run into the problem of trying to explain why the gravity of the Bullet Cluster seems to have decoupled from its visible matter.

As to your 'deeper' speculations. Nobody pretends that the SM of physics is the last word. Indeed there are a rather significant number of arbitrary free parameters. However, we have measured the value of most of them to very precise values, and any 'deeper' (I would argue simpler) theory is going to have to generate those values. The question is whether we can find a theory that provides great enough additional explanatory power that it can predict new phenomena or explain unexplained phenomena. That and one must question whether ANY theory can ever get rid of all free parameters. This is the problem String Theory has, there are still free parameters and it so far hasn't predicted anything that the SM doesn't, at least not that we are equipped to observe (this may be changing). Speculations are wonderful, but it seems to me that where we are now is that we have a warty theory, but one that explains a LOT of stuff quite well. We're going to have to gather a lot more data and do some very clever analysis to work out a more fundamental theory. In essence IMHO all we can really ever hope to do is keep cutting the number of free parameters down. Maybe we can get to 1, that would be a TOE, a Theory of Everything, but it still wouldn't explain WHY its one free parameter has any specific value. "Ultimate Why" is probably a nonsensical question, there are no final answers, as Godel's work on Completeness so aptly suggests.

"The issue of course is that for OTHER situations, things like the Bullet Cluster, the characteristics of the DM and its motion are quite important and things like MOND entirely break down, they simply have no explanatory power. "

I believe they fixed that; at worst, you are still off by only a factor of two, which at that point could be more easily explained by non luminous normal matter. It it's most recent version, TeVeS passes the Bullet test.

And yes, the "gravity works differently" theory is weak on it's surface. But the way I look at it, if there is another dimension involved, then the gravity doesn't have to work any differently at all. It would just be the 'location' of it that has moved.

' "Ultimate Why" is probably a nonsensical question, ' and yet, in the end, is likely the only one that will matter. The answer may be, the "Ultimate Irony".

"The issue of course is that for OTHER situations, things like the Bullet Cluster, the characteristics of the DM and its motion are quite important and things like MOND entirely break down, they simply have no explanatory power. "

I believe they fixed that; at worst, you are still off by only a factor of two, which at that point could be more easily explained by non luminous normal matter. It it's most recent version, TeVeS passes the Bullet test.

And yes, the "gravity works differently" theory is weak on it's surface. But the way I look at it, if there is another dimension involved, then the gravity doesn't have to work any differently at all. It would just be the 'location' of it that has moved.

' "Ultimate Why" is probably a nonsensical question, ' and yet, in the end, is likely the only one that will matter. The answer may be, the "Ultimate Irony".

Actually I would say that the legitimate business of science is more 'useful answers', not answering any kind of ultimate questions. At the edges it is plainly going to verge on more purely philosophical questions, but in the end is there some objective superiority to a theory with one variable vs one with 17 if they produce the same results? I don't think so. One might be said to be more elegant, but I'm not even sure that's guaranteed to be true.

As for your own pet "extra dimensions" gravity theory, I can only say that without a mathematical formulation it doesn't say much, and can't really be evaluated. Even if such was forthcoming it would be outside of my competency to truly evaluate it. I may hold a degree in math, but I'm FAR from being facile with GR and other classical field theories. I once dated a woman who COULD handle that sort of math, I've been taught graphically exactly where my limits are... Likewise I personally cannot evaluate TeVeS, but if it doesn't work as well as CDM then what does it have going for it? More complex gravity or new form of matter, its not clear to me that one is more parsimonious than the other.

[Actually I would say that the legitimate business of science is more 'useful answers', not answering any kind of ultimate questions. At the edges it is plainly going to verge on more purely philosophical questions, but in the end is there some objective superiority to a theory with one variable vs one with 17 if they produce the same results? I don't think so. One might be said to be more elegant, but I'm not even sure that's guaranteed to be true.

Yeah. But you went there first.

I really don't think there are any constants, and the ones we have are placeholders. There are reasons and causes and sub-systems for all of them. The alternative, is that is simply the way the Creator made things. In which case peering behind the curtain any further is a waste of time. So basically, constants are Creationism in your otherwise pure science. I bet you don't like that; I know I don't.

So yeah, all things being equal, and equations working out; one universal constant is far superior to 17. I would count that as 16 steps closer to getting 'there'. Heh. 'There', they have transporters and warp drives, It's a cool place.

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As for your own pet "extra dimensions" gravity theory, I can only say that without a mathematical formulation it doesn't say much, and can't really be evaluated. Even if such was forthcoming it would be outside of my competency to truly evaluate it.

And you don't have to evaluate it, or offer me any proofs or anything. You are certainly competent to casually discuss it, and if it interests you, I hope you do.

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I may hold a degree in math, but I'm FAR from being facile with GR and other classical field theories. I once dated a woman who COULD handle that sort of math, I've been taught graphically exactly where my limits are... Likewise I personally cannot evaluate TeVeS, but if it doesn't work as well as CDM then what does it have going for it? More complex gravity or new form of matter, its not clear to me that one is more parsimonious than the other.

Oh yeah, the former is far preferable to the latter. Not sure about parsimonious, that word is 2 letters past my limit.

What has a wimp done for me lately? What could it do? On the other hand, more complex gravity gives me warp drive. That would be a huge discovery, don't you think? (the gravity, not the warp drive)

[Actually I would say that the legitimate business of science is more 'useful answers', not answering any kind of ultimate questions. At the edges it is plainly going to verge on more purely philosophical questions, but in the end is there some objective superiority to a theory with one variable vs one with 17 if they produce the same results? I don't think so. One might be said to be more elegant, but I'm not even sure that's guaranteed to be true.

Yeah. But you went there first.

I really don't think there are any constants, and the ones we have are placeholders. There are reasons and causes and sub-systems for all of them. The alternative, is that is simply the way the Creator made things. In which case peering behind the curtain any further is a waste of time. So basically, constants are Creationism in your otherwise pure science. I bet you don't like that; I know I don't.

So yeah, all things being equal, and equations working out; one universal constant is far superior to 17. I would count that as 16 steps closer to getting 'there'. Heh. 'There', they have transporters and warp drives, It's a cool place.

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As for your own pet "extra dimensions" gravity theory, I can only say that without a mathematical formulation it doesn't say much, and can't really be evaluated. Even if such was forthcoming it would be outside of my competency to truly evaluate it.

And you don't have to evaluate it, or offer me any proofs or anything. You are certainly competent to casually discuss it, and if it interests you, I hope you do.

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I may hold a degree in math, but I'm FAR from being facile with GR and other classical field theories. I once dated a woman who COULD handle that sort of math, I've been taught graphically exactly where my limits are... Likewise I personally cannot evaluate TeVeS, but if it doesn't work as well as CDM then what does it have going for it? More complex gravity or new form of matter, its not clear to me that one is more parsimonious than the other.

Oh yeah, the former is far preferable to the latter. Not sure about parsimonious, that word is 2 letters past my limit.

What has a wimp done for me lately? What could it do? On the other hand, more complex gravity gives me warp drive. That would be a huge discovery, don't you think? (the gravity, not the warp drive)

Some of the down-voting in this thread is shameful. There are people asking perfectly reasonable questions being modded down to invisibility. Yes, I know that it can't be anti-matter because that interacts with out matter rather too well rather than not at all; but I didn't always know that. At one point I asked what anti-matter was and someone was kind enough to tell me. Not blast me down for not knowing.

I get that some of the posts here may come off as crackpot ideas but others are simply enquiring minds using their imagination and then asking questions based on it. Which is half of what science is. It's not like they're proposing that the extra gravity can be accounted for by your momma, or something.

Some of the down-voting in this thread is shameful. There are people asking perfectly reasonable questions being modded down to invisibility. Yes, I know that it can't be anti-matter because that interacts with out matter rather too well rather than not at all; but I didn't always know that. At one point I asked what anti-matter was and someone was kind enough to tell me. Not blast me down for not knowing.

I get that some of the posts here may come off as crackpot ideas but others are simply enquiring minds using their imagination and then asking questions based on it. Which is half of what science is. It's not like they're proposing that the extra gravity can be accounted for by your momma, or something.

that's the problem with the whole thing - a lot of people use the voting mechanism to say "agree"/"disagree", when it's supposed to be more like "constructive"/"spam"

Some of the down-voting in this thread is shameful. There are people asking perfectly reasonable questions being modded down to invisibility. Yes, I know that it can't be anti-matter because that interacts with out matter rather too well rather than not at all; but I didn't always know that. At one point I asked what anti-matter was and someone was kind enough to tell me. Not blast me down for not knowing.

I get that some of the posts here may come off as crackpot ideas but others are simply enquiring minds using their imagination and then asking questions based on it. Which is half of what science is. It's not like they're proposing that the extra gravity can be accounted for by your momma, or something.

that's the problem with the whole thing - a lot of people use the voting mechanism to say "agree"/"disagree", when it's supposed to be more like "constructive"/"spam"

I disagree! -1! Yeah, its true. I think all the AGW debates have gotten the people interested in technological debates rather trigger-happy with the down votes lately.

that's the problem with the whole thing - a lot of people use the voting mechanism to say "agree"/"disagree", when it's supposed to be more like "constructive"/"spam"

Posts of known-wrong uninformed speculation are indeed "spam" rather than "constructive".

p.s. Towermac, you are spending a lot of hours writing oddball claims that would be better put towards your physics grad school application.

There are more than two buckets, and spam is somebody advertising $1000 a week on the internet by clicking this link. Wrong or uninformed is not spam, and joppek has it right.

Even in the case of the 'could antimatter be dark matter?' caliber post, it didn't take much time for someone to basically say no. You imply that discussion posts have some sort of cost, like this real estate is valuable somehow, days after the story.

'Oddball claims' stings a bit, but okay, I'll own that. What I've been talking about though, is very close to the current alternative theory on DM, so not that odd. But if you can't talk about it, and bang ideas and questions around here, then where? This used to be *the* place for this kind of stuff.

I will admit that I waste too much time in here, but on this particular subject, I simply can't help myself. I think this is the next big, big breakthrough, and we are right on the cusp of it. Or at least I hope so.

As far as physics grad school goes, they'd probably want me to go to college first. Not out of the question at 48, but life's little things, like dropping $1700 into my transmission yesterday, can interfere with big picture plans...

that's the problem with the whole thing - a lot of people use the voting mechanism to say "agree"/"disagree", when it's supposed to be more like "constructive"/"spam"

Posts of known-wrong uninformed speculation are indeed "spam" rather than "constructive".

known-by-who-to-be-wrong?

there are various levels of informed and people like to speculate about things they don't fully understand (otherwise it wouldn't be called speculation). asking stupid questions shouldn't be discouraged - it's much more constructive to say "that idea is wrong and here's why" than to sweep the question under the rug as spam

i often find it much more interesting to discuss topics i know very little of, and asking stupid questions is a great way to learn new things

This really reminds me of the search for luminiferous aether in the late 19th century. If I were to return to theoretical physics (an undergrad degree I have never directly used) I think I would start looking for an error in the standard model that vanishes at "small" scales, much like relativistic effect vanish under the slow-speed low-mass/energy conditions to yield Newtonian physics.

Maybe there is something out there, but to me dark matter seems much too smooth and evenly distributed for something influenced by gravity. If there are wimps, I want to understand why they are not lumpy like the rest of the observable universe.

It seems to me that dark matter behaves exactly as we would expect it to behave; without electromagnetic interactions, matter will not clump together by gravity alone. Dark matter particles will just fly by each other since gravity is not strong enough on such scales to actually change the velocity by any meaningful amount. Given this, I would expect dark matter to stay in very diffuse clouds.

Without interacting electromagnetically, the particles are unlikely to actually hit each other or hit normal matter. Dark matter is probably moving at high speeds, so it would generally not actually be "caught" by any gravitational field short of a black hole. It will go right through the Earth without hitting anything since atoms are mostly empty space with their apparent size only due to electromagnetic repulsion. If a dark matter particle is moving faster than earth's escape velocity, which is slow when considering subatomic particles, it will just keep on going. Also, there isn't really a way for dark matter to slow down much from any initial velocity since it does not produce radiation. As the article states, if dark matter does not interact via the weak force, then it will be close to impossible to detect directly.

Since dark matter appears to be present from gravitational observations, and is not inconsistent with what we would expect, I don't think replacing current theories of gravity is necessary for dark matter. It still is of interest how the dark matter was created and what it is though. Is it still being produced, or is it a left over from super high energy interactions in the early universe? Our current standard model may not be able to explain it ether way, but we can't do much without being able to detect it directly. It may tie into unifying gravity with the other forces, so it is still an interesting thing to study.

So called "Dark Energy" may be a different story though. The inclusion of dark energy does seem like "luminiferous aether" or the complicated Ptolemaic model created in an attempt to keep the Earth rather than the Sun at the center of the Solar System but still explain their motion. Dark energy may exist, but at this point, I think there is still a possibility we are missing something fundamental.

...Since dark matter appears to be present from gravitational observations, and is not inconsistent with what we would expect, ...

Oh, but it is. We would expect to be able to see matter, to interact with it, etc. DM theory tries to account for this lack of interaction, certainly with individually plausible mechanisms (such as immune to all the forces), but when taken as a whole, it's indistinguishable from magic. It does feel like going to an extreme in avoiding extending the theory of relativity.

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I don't think replacing current theories of gravity is necessary for dark matter...

Well, no, nobody's replacing anything. At least I don't think so. Extending is the word here. In the same way that Newtonian physics still work, right up until they don't. You have to get really close to a gravity well, or go really fast, to measure the differences, and find that Newton is not working for you anymore.

GR is not working for us here, and it's time to extend again. But we have two things that shouldn't exist according to GR, and we focus only on the one. Science tells us that a singularity can and does exist at the center of this gravitational anomaly that we are trying to explain. But we don't have a clue as to where that mass/energy goes, what it does, how it's able to exist with infinite density in zero volume. That's magic right there, fully accepted by the scientific community.

...Since dark matter appears to be present from gravitational observations, and is not inconsistent with what we would expect, ...

Oh, but it is. We would expect to be able to see matter, to interact with it, etc. DM theory tries to account for this lack of interaction, certainly with individually plausible mechanisms (such as immune to all the forces), but when taken as a whole, it's indistinguishable from magic. It does feel like going to an extreme in avoiding extending the theory of relativity.

We already know of several particles that behave quite similarly to dark matter; this has already been brought up (neutrons and neutrinos). A neutron can be used to split an atom because it can fly through the electromagnetic field around the atom without interaction and actually get to the nucleus. If it didn't interact by the weak and strong interactions, it probably would fly right by the nucleus also.

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I don't think replacing current theories of gravity is necessary for dark matter...

Well, no, nobody's replacing anything. At least I don't think so. Extending is the word here. In the same way that Newtonian physics still work, right up until they don't. You have to get really close to a gravity well, or go really fast, to measure the differences, and find that Newton is not working for you anymore.

GR is not working for us here, and it's time to extend again. But we have two things that shouldn't exist according to GR, and we focus only on the one. Science tells us that a singularity can and does exist at the center of this gravitational anomaly that we are trying to explain. But we don't have a clue as to where that mass/energy goes, what it does, how it's able to exist with infinite density in zero volume. That's magic right there, fully accepted by the scientific community.

The two are definitely related.

To some extent, I would expect later theories to be more generalizations of current theories, but since the solution to the Dark Matter/Dark Energy inconsistencies my only be resolved by unification of Relativity and quantum mechanics, there could be a massive change rather than just an extension.

I don't think anyone has actually proven the existence of a singularity (actual zero volume) in a black hole. Obviously, relativity predicts a singularity where everything goes to infinity, but this may not actually exist. For a low mass object, electromagnetic repulsion forces prevent collapse. For something more massive, like a neutron star, degeneracy pressures prevents further collapse. In a black hole, nothing is known to prevent collapse into a singularity, but this could be due to incompleteness of the standard model and/or relativity. We can't observe anything on the other side of the event horizon.

Science does not blindly accept dark mater, dark energy, or singularities/infinities. There is a reason why there is so much research into dark matter/energy and black holes; these are just some of the places where the current theories may break, leading to better theories. Note that all of the things which appear to prevent gravitational collapse are based on quantum mechanical theories. For a black hole, quantum mechanics can not be ignored, but I don't know if it is known how it applies. A black hole could really just be a ball of "somethings" with no actual singularity.

...Since dark matter appears to be present from gravitational observations, and is not inconsistent with what we would expect, ...

Oh, but it is. We would expect to be able to see matter, to interact with it, etc.

I think your concern here is more semantic than physical - what your conception of "matter" is. Yours may be a popular lay conception of what "matter" is, but not necessarily a physical one.

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DM theory tries to account for this lack of interaction, certainly with individually plausible mechanisms (such as immune to all the forces), but when taken as a whole, it's indistinguishable from magic. It does feel like going to an extreme in avoiding extending the theory of relativity.

Gravity is a force and dark matter as hypothesized does interact with its own kind, and everything else, via gravity - and possibly one (or more?) of the other forces, but if so then very, very, very weakly. It is not magic. Electrons don't feel the strong nuclear force, does that make them magic too? And what about neutrinos which feel neither the strong nor electromagnetic force? What you seem to be describing is something that doesn't interact with anything, including gravity, which would in fact be nothing as interaction with gravity does not seem to be optional.

...Gravity is a force and dark matter as hypothesized does interact with its own kind, and everything else, via gravity - and possibly one (or more?) of the other forces, but if so then very, very, very weakly. It is not magic. Electrons don't feel the strong nuclear force, does that make them magic too?

No, although they may have seemed that way a few hundred years ago. But electrons are pretty easily detectable, and there's a good, known reason why they don't feel the strong force. Fully explainable and understood (well as much as anything is fully understood).

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And what about neutrinos which feel neither the strong nor electromagnetic force?

And pretty hard to detect too. But we did finally detect them, and more importantly, the standard model depends on them; the whole thing would fall apart without them. So even if we hadn't detected them yet, there would be a hole in the SM, and we would be looking for something.

I understand that line of reasoning does more to support DM than detract from it. And yet to me, a hole in the standard model would be more persuasive than unexplained galactic lensing and rotations.

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What you seem to be describing is something that doesn't interact with anything, including gravity, which would in fact be nothing as interaction with gravity does not seem to be optional.

Well I didn't mean to do that; the whole discussion, and certainly my posts, are about gravity.